1. Field of the Invention
This invention relates to a strain gauge. In more detail, it relates to a strain gauge in which metallic foil pattern sections including a narrow gauge element pattern section formed of a resistance material and capable of changing its resistance in response to a detected strain, at least a pair of wide gauge tab pattern sections connected with gauge leads, and at least a pair of connecting pattern sections capable of electrically connecting the respective ends of said gauge tab pattern sections and said gauge element pattern section, are attached onto a gauge base formed of a flexible insulation material; and said gauge base having said metallic foil pattern sections attached is covered on the surface side with a laminating film. It is a strain gauge for measuring large stains, which can be used also to measure large strains.
2. Description of the Related Art
A strain gauge is used to convert the mechanical strain on the surface of a machine or structure (hereinafter called “an object to be measured”) into a resistance value or electric quantity such as voltage, for accurately measuring the deformation (strain) of each portion of an object to be measured, for quantitatively obtaining the strain distribution of the object to be measured, without breaking it in a state where it is actually used.
A conventional strain gauge used for such an application has a constitution as shown in FIGS. 4 and 5.
FIG. 4 is a plan view typically showing the constitution of a conventional foil gauge type uniaxial strain gauge.
FIG. 5 is a sectional view showing a state where the strain gauge of FIG. 4 is bonded to an object to be measured.
In FIGS. 4 and 5, symbol 1 denotes a gauge base formed of a flexible insulation material. Symbol 2 denotes a narrow gauge element pattern section formed by patterning a conductive foil (metallic foil) by a photo etching technique into a continuously folded-back zigzag pattern with a predetermined length (gauge length) as shown in FIG. 4. The resistance value of the gauge element pattern section changes in response to the strain of the object to be measured 9 transmitted through said gauge base 1.
Symbols 3a and 3b denote a pair of gauge tab pattern sections formed of the same material as that of said gauge element pattern section 2 simultaneously with it and are formed to be wider compared with the width of the strand of the gauge element pattern section 2, to facilitate the connection with gauge leads (lead wires) 4a and 4b. Symbols 5a and 5b denote connecting pattern sections connected at their ends on one side with said pair of gauge tab pattern sections 3a and 3b and connected at their ends on the other side with the respective ends of the gauge element pattern section 2. Symbols 6a and 6b denote soldering portions for electrically connecting the gauge leads 4a and 4b with the gauge tab pattern sections 3a and 3b. Symbol 7 denotes a laminating film covering the surfaces of the gauge element pattern section 2, the gauge tab pattern sections 3a and 3b and the connecting pattern sections 5a and 5b for preventing the deterioration caused by moisture absorption and the damage incurred from outside.
In the case where a strain gauge constituted as above is used for measuring large strains, various problems arise.
First, a general foil type strain gauge has a difficulty that, for example, in the case where it is bonded to an object to be measured and elongated by pulling, the disconnection due to breakage occurs in the region A of FIG. 4 (the gauge tab pattern sections 3a and 3b) at a strain of about 8%, since stress concentration occurs at the boundary portion between the region A (the gauge tab pattern sections 3a and 3b) and the region B (the connecting pattern sections 5a and 5b), namely near the junction between the region A and the region B.
The reasons are considered to be that the region A (the gauge tab pattern sections 3a and 3b) is larger in foil area than the region B (the connecting pattern sections 5a and 5b) and that the region A is in a state of being more unlikely to be elongated than the region B, since the region A is soldered to the gauge leads 4a and 4b. 
So, the applicant proposed a strain gauge as shown in FIG. 4 in the past (Patent Document 1: JP3040684U).
A feature of the strain gauge shown in FIG. 4 and described in Patent Document 1 is that slits are formed for deconcentrating the stress at the portions where the gauge tab pattern sections 3a and 3b are connected with the connecting pattern sections 5a and 5b, to thereby prevent the disconnection due to breakage. This is described below more particularly. The gauge tab pattern sections 3a and 3b are formed to have a circular arc at their ends on the side facing the gauge element pattern section 2, with the hollow face of the circular arc turned toward the gauge element pattern section 2 (leftward in FIG. 4). Further, at the junctions of the gauge tab pattern sections 3a and 3b with the connecting pattern section 5a and 5b, slits 8a and 8b with a depth starting from the ends of the gauge tab pattern sections 3a and 3b on the gauge element pattern section 2 side to reach near the central portions of the gauge tab pattern sections 3a and 3b are formed in the sensitive axis direction.
In the strain gauge constituted as described above and described in Patent Document 1, the regions near the deepest portions 8c and 8d of the slits 8a and 8b where the stress is concentrated are near the central portions of the wide and long gauge tab pattern sections 3a and 3b, hence respectively having a wide area. So, the stress is deconcentrated to eliminate the possibility of the disconnection due to breakage otherwise caused by stress concentration.
Therefore, the gauge element pattern section 2 receiving the strain of an object to be measured, through the gauge base 1 receives the strain and changes its resistance in response to the strain. Though the strain limit value of a conventional strain gauge used for measuring large strains was less than 10%, the strain gauge shown in FIG. 4 can measure strains of more than about 15% without causing the disconnection due to breakage.
[Patent Document 1] JP3040684U
However, in recent years, it is demanded to develop a strain gauge capable of measuring large strains of more than said strain limit of 15%.
So, the present inventors grappled with various problems, for meeting the demand.
At first, the adhesive strengths of presently commercially available adhesives were tested, and for example, “CC-36,” a commodity sold by the present applicant did not have any problem for strains of 20% or more, and was effective also for large strains of 30% or more. However, the strain limit of bonded strain gauges per se could not exceed 15%.
Further, the pattern of the metallic foil used to form the gauge element pattern, etc. and the foil material were experimentally changed, but no significant improvement could be seen.
Furthermore, for enhancing the adhesive strength of the gauge base to an object to be measured, various treatments of grinding the bonded surfaces were tried, but though some effects of improvement could be seen, the trials could not find a fundamental solution.
Moreover, to enhance the adhesiveness, it was also considered to enlarge the adhesive area of the gauge base, but the strain gauge as a whole became so large as to limit the applicable objects to be measured as another problem.
This invention has been completed in view of the situation as described above. A first object of this invention is to provide a strain gauge for measuring large strains, which can be used to measure large strains of more than 20%. A second object is to provide a strain gauge for measuring large strains, which is unlikely to be separated from the object to be measured even if a large strain acts. A third object is to provide a strain gauge for measuring large strains, which can be kept very small in size. A fourth object is to provide a strain gauge for measuring large strains, which is easy to produce, easy to bond and inexpensive.